Heat and aging effects on thyroid function of male rats.

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Abstract
One hundred male rats were separated into 3 groups at 21 days of age: group A was reared at 28 C (thermoneutral zone) and fed ad libitum, group B was reared at 34 C and fed ad libitum, and group C, reared at 28 C and control fed (amount consumed by group B). Thyroid I131 release rates were measured monthly beginning at 34 days of age up to 187 days, then on alternate months throughtout their lives. Thyroid I131- release rate constants tended to be depressed at the elevated temperature (34 C) up to 40 days, but thereafter increased to a level significantly higher than in group A or C. This higher thyroidal I131-release rate of the heat-exposed animals persisted throughout their lifetime. Thyroid sections examined histologically suggested the glands in older animals reared at 34 C to be hyperactive whereas the 2 groups reared at 28 C demonstrated characteristics associated with normal thyroid glands. Oxygen consumption (milliliters per hour gram of body weight) of these groups was not significantly different throughout their lifetime except at the younger age of 42 days. At this age, the values for the animals at 34 C were significantly lower than those for the 28 C control-fed and ad libitum-fed groups.

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